Methionine and cysteine are sulfur-containing proteinogenic amino acids. Methionine is an essential amino acid for monogastrics and finds a variety of applications in the food and medical industries. For example, methionine is used as an additive in animal feed and foods and as an ingredient in parenteral nutrient solutions and medicines.
Physiologically, methionine can be a precursor for choline (lecithin) and creatine and is used as a starting material for the synthesis of cysteine and taurine. Typically, chemically synthesized DL-methionine has been used in the market for additives for animal feed.
L-methionine has been produced by direct fermentation. However the low solubility of L-methionine can cause formation of crystals during direct fermentation to produce L-methionine. These crystals make it difficult to agitate the culture medium and may decrease fermentation yield. The sulfur required to produce L-methionine by direct fermentation can also adversely affect bacterial growth.
WO 2008/013432 describes a two-step process for producing L-methionine which includes the fermentative production of an L-methionine precursor (e.g., O-acetylhomoserine (OAHS) or O-succinyl homoserine (OSHS)) followed by the enzymatic conversion of the precursor to L-methionine.
When OAHS is the precursor, methyl mercaptan (CH3SH) and OAHS react to produce acetate and methionine. When OSHS is the precursor, CH3SH and OSHS react to produce succinate and methionine.
The CH3S-residue of methyl mercaptan is substituted with succinate or acetate residue of OSHS or OAHS to produce methionine. Methyl mercaptan (CH3SH) can be added in different forms during the reaction.